In audio devices, radio reception in the AM frequency range is normally achieved by providing an internal ferrite antenna. Such ferrite antennas are configured for a predetermined frequency band in the radio spectrum. The ferrite antenna provides the functions of both an antenna receiving the magnetic part of the electromagnetic wave, and a resonant circuit with an additional capacitance. The first stage of a subsequent antenna amplifier may then have a wide band configuration, while selectivity is obtained in the second amplifier stage.
Such ferrite antennas may be disadvantageous in that internally generated electromagnetic fields (e.g. from a central processing unit (CPU), a microcomputer (μC) or the like) are picked up as noise. Additionally, the physical dimensions of the internal ferrite antenna are a key factor for sensitivity of AM reception. Increased miniaturization of audio devices with radio receivers requires smaller ferrites, which leads to undesirable decreases in sensitivity of radio reception.
By putting the AM antenna outside the housing, sensitivity can be increased and additional area can be made available on a printed circuit board (PCB) by removing the ferrite. This freed area can be used for other circuit elements.
One placing option for external radio antennas may be a headset of the audio device. This is suggested for example in the US 2005/0285799 A1 which discloses a headset loop antenna implemented by loop sections which include inductors and wherein conductors to the earplugs contain ferrite beads. Each conductor in the loop section forms a matching element in that it matches to the desired reception frequency of the loop antenna. The loop antenna segments are coupled to one another and to a conductive antenna lead section at a Y-type coupler. Alternatively, the two antenna loop segments may be joined directly at a plug of the headset.
However, to emulate the presence of a ferrite antenna and/or to tune the loop antenna to the desired frequency range, proper selection of the inductors within the antenna loop configuration is crucial for matching the antenna to the receiver circuit and frequency range.